91 (16.8%) of 541 consecutive patients investigated for chest pain or after recent uncomplicated myocardial infarction had a rise in diastolic blood pressure (DBP) of more than 15 mm Hg during a symptom-limited treadmill test. 63 also had electrocardiographic evidence of ischaemia, but 28 did not have 1 mm ST segment depression, of whom 24 had angiographic evidence of more than 70% stenosis of two or more major coronary arteries. 55 of these 91 patients underwent coronary artery bypass surgery; repeat angiography in 22 at 12 months showed an improved left ventricular ejection fraction in 18 who had a normal postoperative DBP response, but no change in ejection fraction in the 4 who still had an abnormal rise in DBP on exercise. Exercise-induced ischaemia may cause a reversible fall in cardiac output that sometimes leads to reflex vasoconstriction and a rise in DBP before a fall in systolic blood pressure or ECG evidence of ST segment depression. An abnormal DBP response to exercise may identify some patients at high risk of myocardial infarction who might otherwise have false-negative exercise tests.